1. Field of the Invention
The present invention is directed to an actively shielded, transverse gradient coil arrangement for a nuclear magnetic resonance tomography apparatus, of the type wherein each gradient coil being composed of two sub-coils and each sub-coil being composed of a primary and a secondary coil radially spaced from one another, the secondary coil lying on a larger radius than the primary coil, and the primary and secondary coil generating a linear magnetic field gradient in the center of an examination volume.
2. Description of the Prior Art
European Application 0 216 590, for example, discloses such an actively shielded gradient coil arrangement of the above general type.
As is known, a spatial resolution of the nuclear magnetic resonance signals ensues in nuclear magnetic resonance tomography by generating a uniform, static basic field on the order of magnitude of 1T with magnetic field gradients superimposed on it. The principles of such imaging are explained, for example, in the article by Bottomley, "NMR Imaging Techniques and Applications--A Review", in Review of Scientific Instrumentation 53, 9, 9/82, pp. 1319-1337. For spatial resolution in three dimensions, magnetic field gradients must be generated in three directions that are preferably perpendicular relative to one another. The conventional structure of gradient coils used to generate such gradient fields (not actively shielded in this case) shall be set forth below with reference to FIGS. 1 and 2. An examination volume 4 with a coordinate axes x, y, z representing the directions of the respective gradients is shown in each of FIGS. 1 and 2.
FIG. 1 schematically shows a conventional arrangement of transverse gradient coils for generating a magnetic field gradient G.sub.y in the y-direction. The gradient coils 2 are implemented as saddle coils that are secured on a carrying tube 1. A substantially constant magnetic field gradient G.sub.y is generated in the y-direction within the spherical examination volume 4 by the conductor sections 2a. The return conductors generate only slight magnetic field components in the examination volume 4 due to their size and distance from the examination volume 4; these magnetic field components are often left out of consideration when designing the gradient coil.
The gradient coils for the x magnetic field gradients are identical to the gradient coils 2 for the y magnetic field gradients and are merely turned by 90.degree. in azimuthal direction on the carrying tube 1. For clarity they are therefore not shown in FIG. 1.
The conventional (axial) gradient coils 5 for the magnetic field gradients in the z-direction are schematically shown in FIG. 2. The coils are annularly implemented and arranged symmetrically relative to the center 3 of the examination volume 4. Since the two individual coils 3a and 3b respectively have current flowing in opposite directions therein as shown in FIG. 2, they cause a magnetic field gradient in the z-direction.
Given actively shielded gradient coils as shown, for example, in the aforementioned European Application 0 216 590, the gradient coils 2 of FIG. 1--which are referred to as primary coils-are surrounded by further, identical coils that are likewise arranged on a cylinder surface that, however, has a larger diameter than the coil carrier 1. These further coils are referred to as secondary coils and have current therein in the opposite direction when compared to the gradient coil arrangement 2. This achieves a gradient field that is substantially attenuated toward the outside, and thus the induction of eddy currents in surrounding, metallic parts is largely suppressed. An unavoidable attenuation of the gradient field thus also occurs in the examination volume 4 and must therefore be compensated by larger numbers of turns and/or higher currents.
In order to avoid image distortions, high demands are made on the linearity of gradient fields; these demands cannot be satisfied with the schematically illustrated, simple conductor structures of FIGS. 1 and 2. The transverse gradient coils are thereby complicated in terms of design. German OS 42 03 582, corresponding to U.S. Pat. No. 5,309,107, discloses a numerical method for calculating more complex coil geometries (conductor orientation) with which the gradient coils can be optimized in many ways by formulating boundary conditions.
Given specific pulse sequences, for example echo planar imaging (EPI), rapidly oscillating gradient fields having a high field strength are required. Uncontrollable physiological reflexes of the examination subject having more or less painful side effects can thereby locally occur due to interactions with nerves and muscles. Although the biophysical mechanisms of these so-called stimulations are extremely complex, the triggering factor thereof is always the chronological change of the magnetic flux .PHI. to which the person under examination is exposed. Since, for example, it is advantageous for EPI imaging to employ high gradient fields given frequencies of approximately 1KHz, one must generally expect high values for d.PHI./dt.
The relationship between useful gradient and flux density in the coil volume is comparatively unfavorable in transverse gradient coils. FIG. 3 shows the field lines of a coil arrangement for generating a y-gradient in the plane x =0. The concentration of the field lines that corresponds to a high magnetic flux density can clearly be seen outside the imaging volume. Whereas the major part of the field generated by the coil is conducted around the examination volume, the useful field itself seems very small in relation. Although this situation cannot be entirely avoided for physical reasons, one can nonetheless attempt to optimize the ratio of useful field to the gradient fields of the examination volume. When one succeeds in achieving this, the problems of the stimulation of physiological stimuli given rapidly switched gradients with high amplitude are also alleviated.
Such an optimization can be fundamentally approached with the method from the aforementioned German OS 42 03 582 and U.S. Pat. No. 5,309,107. For example, an objective is formulated with reference to the optimization procedures so that the maximum deviations of the rated field within the examination volume are prescribed and the radial field course in the outer space is simultaneously minimized. An improvement in the ratio of the useful field to other coil fields can in fact be fundamentally achieved with this method, but only within relative narrow limits. This arises from the fact that one does not have adequate degrees of freedom for an optimization due to the separate construction of the primary coil and the secondary coil on two cylindrical surfaces.